The spatially resolved near-field radiative heat transfer between a nanoscale probe and a substrate is studied in the fluctuational electrodynamics framework within the dipolar approximation. It is shown that the introduction of a thin polar film atop a nondispersive substrate can lead to both an enhancement and a lateral focusing of the heat exchange. The influence of the probe-substrate separation, film thickness, and substrate permittivity is analyzed, revealing that the effect originates from near-field interactions governed by the interplay between film-induced modifications of electromagnetic mode dispersion and the distance-dependent coupling strength. The results highlight a viable route toward the active control of local radiative heat transfer at the nanoscale.

Betatron x rays generated from laser-wakefield accelerators are a promising source for imaging dynamic processes in materials. Here, we present proof-of-concept imaging of microstructural evolution in a hypermonotectic $\mathrm{Al}$-$\mathrm{Bi}$ alloy, which consists of liquid $\mathrm{Bi}$ particles in a solid $\mathrm{Al}$ matrix. The images capture the elongation and fragmentation of $\mathrm{Bi}$ particles upon isothermal annealing. Because of the femtosecond time scale of the betatron source, the images are not subject to motion blur, whereas the accessibility of the source allows for studies of long-term processes such as annealing. The high-resolution data reveal that the evolution of $\mathrm{Bi}$ particles is mediated by an interplay of grain-boundary wetting and morphological instability, in stark contrast to an earlier proposal for (inverse) coarsening.

Capacitive sensing has been extensively utilized in high-precision measurements, including gravitational-wave detection. Compared to conventional open-loop schemes, the closed-loop resonance-tracking method significantly enhances the measurement dynamic range. Previous analyses focused primarily on the linear and adiabatic regimes. Here, we extend the theoretical framework into two unexplored regimes: (i) the linear yet nonadiabatic regime, and (ii) the nonlinear but adiabatic regime. These extensions are crucial for applications requiring both high sensitivity to minute displacements and the capability to track large-scale, slow variations. Beyond theoretical modeling, we implement a proof-of-principle experiment utilizing a hybrid analog-digital circuit, validating the predictions of our extended theoretical model. The developed modeling and design principles are generalizable to other resonance-enhanced measurement systems, potentially broadening their applicability in precision metrology.

Space-division multiplexing using multimode optical fibers has been applied to quantum-level signals with time-bin and phase encoding, achieving megaqubits per second over 8 km of few-mode fiber. Time windowing is efficiently employed to eliminate modal crosstalk between quantum channels. The dead time of the single-photon detectors in each quantum channel, typically regarded as a bottleneck in quantum transmission, is synchronized with the time-window intervals, thereby optimizing the detection rates.

Inversion symmetry breaking plays a pivotal role in various quantum phenomena and is fundamental for the generation of spin-orbit torque (SOT), which is essential for the advancement of next-generation spintronic technologies. By use of a ferromagnetic single layer exhibiting crystal inversion asymmetry, the efficiency of SOT magnetization switching has been significantly increased at low temperatures. To extend its applicability to room temperature, we engineered an epitaxial ferromagnetic half-Heusler alloy $\mathrm{Ni}\mathrm{Mn}\mathrm{Sb}$ single layer with an intrinsic bulk noncentrosymmetric structure on an $\mathrm{Mg}\mathrm{O}$(100) substrate using magnetron sputtering. Through a comprehensive analysis of the angular correlation of the planar Hall resistance $R_{\mathrm{PH}}$, the SOT efficiency was found to be 16.8 Oe/(${10}^6\mathrm{A}/{\mathrm{cm}}^2$), which significantly exceeds that of conventional heavy metal/ferromagnet bilayer structures and a $\mathrm{Ni}\mathrm{Mn}\mathrm{Sb}$ single layer grown on $\mathrm{Ga}\mathrm{As}$ substrates. The high efficiency can be attributed to the intrinsic crystallographic symmetry breaking and half-metallic properties of $\mathrm{Ni}\mathrm{Mn}\mathrm{Sb}$, and the extremely small lattice mismatch (0.15%) with $\mathrm{Mg}\mathrm{O}$. Our results provide compelling experimental evidence advocating the use of a $\mathrm{Ni}\mathrm{Mn}\mathrm{Sb}$ single layer in practical SOT-based devices, advancing the development of high-performance, energy-efficient spintronic applications.

The electrical control of pure spin current remains a central challenge in spintronics, particularly in time-reversal symmetric systems composed of nonmagnetic elements, where spin and electric fields interact only indirectly. In this work, we develop a theoretical framework for electrically tuning pure spin photocurrent in two-dimensional materials with time-reversal symmetry via a gate electric field. Through theoretical analysis, we demonstrate that in systems with spin-orbit coupling and in-plane mirror symmetry, an out-of-plane electric field induces spin splitting and reversal in the band structure near the Fermi energy, enabling magnitude control and direction reversal of the pure spin photocurrent. To validate this mechanism, we perform first-principles calculations on germanene, an experimentally realized two-dimensional material. Beyond amplitude modulation, we reveal that reversing the direction of the applied electric field leads to a corresponding reversal of the pure spin photocurrent. Furthermore, we show that the pure spin photocurrent can be tuned by varying the photon energy and the incident angle of light, providing additional degrees of control over spin transport. These findings establish a robust strategy for electric-field-controlled pure spin transport in two-dimensional materials, offering possibilities for the development of optospintronic devices.

Accelerometers offer motion-sensing capabilities across a wide range of areas, enabling navigational awareness in consumer goods and defense applications, and playing a key role in monitoring and control systems. To date, on-chip accelerometers have largely utilized a single device layer or substrate as a test mass. This constrains the test mass to the dimensions and density of the device layer or substrate, ultimately limiting the sensitivity of the device. We demonstrate an alternative approach, which utilizes a pick-and-place bonding technique to increase the test mass of an on-chip accelerometer. By bonding a high-density platinum sphere to a nanomechanical silicon-nitride trampoline membrane, we achieve a quality factor of 1900 in air with 95-mg test mass, corresponding to a thermomechanical noise-limited acceleration sensitivity of 0.8 $\mathrm{n}g/\sqrt{\mathrm{Hz}}$. We optically probe the device’s response to applied accelerations with increasing levels of acoustic and vibration isolation, measuring a peak sensitivity of 5.5 $\mathrm{n}g/\sqrt{\mathrm{Hz}}$ at 117 Hz in air, limited by environmental vibrations. This represents the best peak sensitivity reported using a chip-integrated test mass.

Studies of superdirective dielectric lens antennas are reported emphasizing unidirectional properties. The developed antennas are based on the higher-order transverse electric and magnetic modes present in a multilayered sphere realized with high-permittivity dielectrics. The superdirective properties of the lenses are empowered by exciting them with a basic unidirectional mixed-multipole antenna. Genetic algorithm optimization was employed to identify configurations that yielded both large directivity and front-to-back ratio outcomes. The analytical evaluations of several example systems of spheres having different outer radii, numbers of layers, and permittivities are described. Their radiated field patterns are presented along with comparisons with known directivity bounds to confirm their superdirective performance characteristics. Subsequent numerical assessments with commercial full-wave finite-element simulations of more realistic versions of several selected examples illustrate their potential for experimental realization.

The Heisenberg exchange constant is crucial for magnetic nanoobject switching behavior. This work estimates the exchange stiffness of $L{1}_0$-ordered Fe-Pd thin films with perpendicular anisotropy, finding that the standard models fail to explain deviations in spin-wave resonance dispersion. Negative surface pinning or epitaxial stress-induced magnetoelastic gradients emerge as competing explanations for the discrepancy. Using ferromagnetic resonance at 112 GHz and micromagnetic simulations, the stiffness is determined as 8.2 ± 0.1 pJ/m, with implications for magnetic memory as low as 10-nm bit cells.

Proximitized $\mathrm{Ge}$ quantum wells have emerged as a platform for studying Andreev bound states (ABSs) due to their expected strong spin-orbit interaction and high mobility. Here, we used microwave spectroscopy techniques to investigate ABSs in Josephson junctions (JJs) realized in proximitized $\mathrm{Ge}$ quantum wells. Spectroscopic signatures observed in a 350-nm junction indicated the presence of multiple ABSs and were reproduced with a model including finite-length effects. The ABS spectra measured for a $1.2$-$\text{μ}\mathrm{m}$ junction were explained by a model including three ABSs in two conduction channels and finite Coulomb interaction. Our work highlights the importance of interactions in JJs and serves as a basis for understanding and manipulating ABSs in $\mathrm{Ge}$-based hybrid devices.

In this study, we examine the supercurrent diode effect (SDE) in mesoscopic superconducting weak links formed by asymmetric Dayem bridges. These planar metallic constrictions, which naturally exhibit Josephson behavior, offer a fundamental platform for investigating nonreciprocal transport phenomena in a regime where the bridge width aligns with the superconducting coherence length ($W\sim \xi$). The foundational concept is inspired by the Tesla valve, a classical fluidic device that achieves flow rectification through interference and turbulence between fluid streams enabled by geometric asymmetry. Analogously, we demonstrate that spatial asymmetry within superconducting structures can result in rectification due to the polarity-dependent interaction between transport and screening currents. By implementing controlled geometric defects at the junction between the constriction and superconducting leads, we induce current crowding and disrupt spatial inversion symmetry, thus facilitating directional switching behavior. Experimental results indicate a linear-in-field rectification regime at low magnetic fields, driven by the interaction between transport and screening currents, which is succeeded by complex vortex dynamics within the superconducting banks at elevated fields. Time-dependent Ginzburg–Landau simulations replicate significant features of the experimental observations and substantiate the influence of both screening currents and rearrangements of Abrikosov vortices. A comparative study across various geometries highlights the crucial role of defect shape and spatial confinement in determining the rectification efficiency, revealing a minimum threshold in bridge width below which crowding-induced SDE is significantly reduced. Our findings advocate for mesoscopic Dayem bridges as a flexible platform for designing and controlling superconducting diode functionalities.

The rapid and precise inverse design of an arbitrary Mie scatterer with functional response facilitates various applications in multiple disciplines. Compared with traditional time-consuming optimization methods, deep learning (DL) is an effective approach in predicting the electromagnetic multipole response and realizing precise inverse design. However, current DL-assisted inverse design methods for arbitrary Mie scatterers suffer from limitations of prediction accuracy and generalizability due to the inherent constraints of incorporating induced-current-based Cartesian electromagnetic multipole expansion. Herein, we propose a deep neural network (DNN) based on dataset generated from spherical-harmonic-based electromagnetic multipole expansion, enabling precise and rapid forward prediction as well as inverse design of an arbitrary Mie scatterer with target electromagnetic multipole responses. The proposed DNN can not only predict the scattering cross section (SCS) of different electromagnetic multipoles, but also accurately resolve complex-valued multipole coefficients and far-field radiation patterns, simultaneously. Microwave experiments further consolidate the accuracy of the proposed inverse design method. It is anticipated that the demonstrated spherical-harmonic-based DNN could pave a new way for the inverse design of meta-atoms with versatile electromagnetic functionalities.

Spin estimation plays a crucial role in quantum metrology, particularly in systems like atomic magnetometers where atomic spin ensembles transduce magnetic-field information into measurable optical signals. However, precise spin estimation remains challenging, especially in weak polarization schemes where signals are often obscured by both classical and quantum noise. Conventional fitting methods for spin signal and variance estimation are computationally intensive, lack real-time capabilities, and exhibit sensitivity to stochastic fluctuations and data-acquisition parameters. To address these challenges, we propose and demonstrate the implementation of a Kalman filter (KF) combined with a Rauch-Tung-Striebel (RTS) smoother for enhanced spin estimation across various polarization conditions. Our comprehensive analysis of spin conditional variance estimation reveals that the KF method achieves a remarkable 4.92-dB noise suppression compared to conventional fitting methods, while the RTS smoother provides further enhancement with 6.94-dB suppression that enables quantum noise squeezing observation. These findings offer new tools for quantum noise analysis and signal recovery, offering significant potential for advancing the performance of quantum-enhanced sensors and other precision measurement technologies.

Biological systems rely on ions and molecules as information carriers rather than electrons, motivating the development of devices that interface with biochemical systems for sensing, information processing, and actuation via spatiotemporal control of ions and molecules. Iontronics aims to achieve this vision by constructing devices composed of ion-conducting materials such as polyelectrolyte hydrogels, but advancing beyond simple single-stage circuit configurations that operate under steady-state conditions is a challenge. Here, we propose and model more complex ionic circuits, namely bistable flip-flop and ring oscillators, consisting of multiple ionic bipolar junction transistors (IBJTs). We begin by modeling and characterizing single IBJTs using both a simplified one-dimensional Nernst-Planck model and a more-detailed two-dimensional Poisson-Nernst-Planck model, elucidating the effects of geometry, size, and fixed charge on the IBJT performance and response time. The one- and two-dimensional models exhibit good agreement, indicating negligible transverse inhomogeneities. In addition, these models show that reducing the base width improves current amplification, a behavior analogous to electronic bipolar junction transistors. Building on this understanding, by using the IBJTs as voltage inverters and buffers, we design and model more complex ionic circuits that dynamically change their states in response to ionic signals. Specifically, we demonstrate that the ionic flip-flop retains one-bit memory and that the ring oscillator achieves autonomous periodic self-oscillation without an external clock. Our work provides a foundation for designing dynamic iontronic circuitry using ionic conductors, enabling biochemical signal processing and logic operations based on ionic transport.

Terahertz technology, a cornerstone of next-generation high-speed communication and sensing, has long been hindered by impedance mismatch challenges that limit device performance and applicability. These challenges become particularly pronounced when ultrasensitive two-dimensional (2D) materials are employed as the detecting element in the terahertz range, further complicating their integration in real-world applications. Furthermore, conventional antenna designs often fail to provide adequate matching across the broad terahertz spectrum. In this work, we tackle these challenges using a procedural generation algorithm to design terahertz broadband antennas that satisfy specific performance criteria. Namely, the developed inverse design methodology enables customization for the target impedance value, bandwidth, and contact topology requirements. The proposed antenna achieves an improvement of up to 40% in power transfer efficiency, compared with traditional bow-tie antennas, under realistic operating conditions. High-fidelity electromagnetic simulations validate these results, confirming the design’s practicality for terahertz applications. This work addresses critical limitations of existing antenna designs and advances the feasibility of high-frequency applications in both communication and sensing.

The nitrogen-vacancy (NV) center in diamond is a prime candidate for quantum sensing technologies. Here, we present a fully integrated and mechanically robust fiber-based endoscopic sensor with a tip diameter of $1.25\mathrm{mm}$. On its tip, a direct laser-writing process is used to secure a diamond containing NV centers above the fiber’s core inside a polymer structure. Additionally, a metallic direct laser-written antenna structure next to the fiber facet allows efficient microwave manipulation of NV-center spins. The sensor achieves a shot-noise-limited magnetic-field sensitivity of $5.9\mathrm{nT}/\sqrt{\mathrm{Hz}}$ using a $15\text{−}\text{μ}\mathrm{m}$-sized microdiamond at a microwave power of $50\mathrm{mW}$ and an optical power of $2.15\mathrm{mW}$. Using lock-in techniques, we measure a sensitivity of $51.8\mathrm{nT}/\sqrt{\mathrm{Hz}}$. Furthermore, we introduce a dual-fiber concept that enables, in combination with a direct laser-written structure, independent guiding of excitation and fluorescence light and thus reduces background autofluorescence. Moreover, controlled guiding of excitation light to the diamond while avoiding sample illumination may enable operation in light-sensitive environments such as biological tissue. While the demonstrated sensitivity is achieved using a single-fiber configuration, the dual-fiber approach provides a path toward integrating smaller diamonds, where autofluorescence would otherwise limit performance. We demonstrate the capability of vector magnetic-field measurements in the type of magnetic field used in state-of-the-art ultracold quantum gas experiments, opening a potential arena in which high resolution and high sensitivity are required.

Silicon nitride is one of the most promising and mature integrated photonics platforms, providing a gateway to the development of cutting-edge devices based on photonic integrated circuits (PICs) with enhanced performance capabilities. For a wide range of applications, integrated heaters remain the preferred method for controlling PICs through the thermo-optic effect. Here, we demonstrate in detail the fabrication process and the characterization of heaters based on a ${\mathrm{Sc}\mathrm{N}}_x$/${\mathrm{Nb}/\mathrm{Sc}\mathrm{N}}_x$ stack. The proposed approach addresses both of the main issues that cause heater failures: electromigration and oxidation (oxygen diffusion). The findings of the experimental investigation suggest that the proposed ${\mathrm{Sc}\mathrm{N}}_x$/${\mathrm{Nb}/\mathrm{Sc}\mathrm{N}}_x$ heaters exhibit a critical current density $j_c\sim 100{\mathrm{GA}/\mathrm{m}}^2$ and are capable of heating silicon nitride waveguides by more than $200{}^{\circ }\mathrm{C}$, providing a specific optical path change per heater length $\mathrm{\Delta }{L}_{\mathrm{optical}}/L_{\mathrm{heater}}\sim 5\text{μ}\mathrm{m}/\mathrm{mm}$ for silicon nitride waveguides in the C band. Thanks to the elevated reliability level and remarkably high efficiency, the proposed ${\mathrm{Sc}\mathrm{N}}_x$/${\mathrm{Nb}/\mathrm{Sc}\mathrm{N}}_x$ heaters potentially enable a significant reduction in the PIC footprint, without compromising performance and robustness. Additionally, we carried out a rough experimental assessment of the magnitude of thermal crosstalk.